Re: st: Heckman Procedure

[Annie Zhang <annieclairezhang@gmail.com>]

Thanks Sergiy,

When I re-run using the code below, fa is not dropped.

heckman cash fa, select(cash=age age_sq sex logfum) twostep


On Tue, Jun 11, 2013 at 2:59 PM, Sergiy Radyakin <serjradyakin@gmail.com> wrote:
> Annie,
> Stata manual is very clear that "The selection equation should contain
> at least one variable that is not in the outcome equation." So perhaps
> you need to go back to the drawing board and decide HOW you want to
> correct for selection. The problem is not in how to specify the
> command to Stata, but how to model the phenomenon you study. Perhaps
> people living closer to the cities are more likely to get financial
> advice? Or those ones with radios? or ones that have gone through
> bankruptcy procedure? Check what other information you can find. Or
> wait for the guys in Europe to wake up in a couple of hours. Best,
> Sergiy
>
> On Mon, Jun 10, 2013 at 10:33 PM, Annie Zhang
> <annieclairezhang@gmail.com> wrote:
>> Hi Sergiy,
>>
>> Thanks so much for your reply. I don't have an instrument variable for
>> the fa (financial advice) variable.
>> The issue is that:
>>
>> Age, gender, wealth affects probability of receiving financial advice.
>> Financial advice affects asset allocation (variable- cash).
>>
>> Therefore I need to correct for self-selection bias that receiving
>> advice may have on asset allocation, given then certain age, gender,
>> wealth affect probability of receiving advice.
>>
>> My main multivariate regression is:
>>
>> reg cash fa age sex logfum
>>
>> so I wanted to run heckman to correct for self-selection bias using:
>>
>> Heckman cash fa age sex logfum, select(fa=age sex logfum) twostep
>>
>> Since fa is being dropped, can you suggest anyway to run heckman that
>> will work in my situation?
>>
>> Thank you so very much!!
>>
>> Kind Regards,
>> Annie
>>
>>
>>
>> On Tue, Jun 11, 2013 at 2:27 PM, Sergiy Radyakin <serjradyakin@gmail.com> wrote:
>>> Annie, Stata is doing the right thing as your variable fa is bringing
>>> no additional explanatory power to the model since it is a linear
>>> combination of your variable age sex and logfum from the first stage,
>>> and thus the separate effect of it can't be identified. Recall that
>>> the coefficient on a variable is the effect of change of that var
>>> holding all other constant (ceteris paribus). Can your fa change when
>>> age,sex, and logfum are fixed? Did you forget perhaps something like
>>> an instrument? Best, Sergiy
>>>
>>> On Mon, Jun 10, 2013 at 10:05 PM, Annie Zhang
>>> <annieclairezhang@gmail.com> wrote:
>>>> Hi there,
>>>>
>>>> When using the heckman procedure I want to use the independent
>>>> variable in the first stage as well as make it a dependent variable in
>>>> the selection model, however stata will drop the variable due to
>>>> collinearity each time.
>>>>
>>>> How can I use the heckman procedure in this case?
>>>>
>>>> My model is
>>>>
>>>>
>>>> heckman cash fa age sex logfum, select(fa=age sex logfum) twostep
>>>>
>>>> And the 'fa' variable keeps dropping.
>>>>
>>>> Thanks for your help!
>>>>
>>>> Kind Regards,
>>>> Annie
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